Elemental sulfur and sulfur compounds are naturally present in many petroleum crude oils. The amount of sulfur varies over a wide range; for example, crude petroleum from West Texas contains about 0.2 weight percent sulfur whereas crude petroleum from Saudi Arabia contains about 5.0 weight percent sulfur. There is also a wide variety of sulfur containing compounds present in the crude. These compounds range from hydrogen sulfide, which is a gas at room temperature, to heavy sulfur compounds that are released at over 1000xc2x0 F. (538xc2x0 C.).
Sulfur compounds are unlikable because of their disagreeable odors and because they oxidize to sulfur dioxide or hydrogen sulfide which are corrosive. The corrosive nature of sulfur compounds contributes significantly to the costs of construction, operation and maintenance of a petroleum refinery. If present in hydrocarbon products, sulfur compounds may cause problems in gasoline engines and can play a substantial role in environmental pollution. Furthermore, sulfur containing crude oil can generate hydrogen sulfide and other sulfur containing gases during transportation and handling which poses a serious health hazard to workers in the immediate area around the crude. Consequently, crude oils that emit sulfur species have a low market value.
When crude petroleum is processed in an oil refinery, one of the first steps is usually to separate the crude into various products based on boiling points. The unit typically used for this separation is a distillation column operated at atmospheric pressure and is commonly referred to as the crude still. The modern oil refinery processes yield a variety of useful fuels and desirable petroleum products, such as lower-boiling gasoline, to middle distillate fuels such as kerosene and diesel oil, to fuel oil for heating, and to higher-boiling waxes and heavy oils such as lubricating oil and asphalt products. The separation of the hydrocarbons also separates the sulfur compounds so that the lower-boiling hydrocarbons contain lower-boiling sulfur compounds and higher-boiling hydrocarbons contain higher-boiling sulfur compounds.
The sulfur compounds are undesirable in every hydrocarbon product coming off the crude still; however, the cause of the undesirability varies from fraction to fraction. It follows that, while most fractions are preferably treated as they come off the crude still to somewhat counteract the bad effects of the sulfur compounds, the preferred method of treatment varies from fraction to fraction.
An enormous amount of technology has developed relating to sweetening and desulfurizing gasolines and other petroleum stocks depending upon the particular type of sulfur compound to be removed. There are five basic processes that have been taught for sweetening and/or desulfurizing: oxidation reactions, solvent extraction, adsorption, metal catalysis, and hydrodesulfurization.
Oxidation reactions can be used to sweeten sour hydrocarbon streams by adding a chemical reagent that oxidizes sulfur components to form sulfides and/or disulfides, usually in the presence of metal reactants. These processes commonly add undesirable metal compounds that remain in the hydrocarbon stream and always require the burdensome step of phase separation. (Jeanblanc, et al., U.S. Pat. No. 5,961,820; Schwarzer et al., U.S. Pat. No. 4,548,708).
The solvent extraction process desulfurizes sour hydrocarbons by extracting the sulfur components from the hydrocarbons with a suitable solvent that is immiscible with the hydrocarbons. All of the solvent extraction processes suffer from the disadvantage of requiring a phase separation. (Forte, U.S. Pat. No. 5,582,714; Horii, et al., U.S. Pat. No. 5,494,752).
The adsorption process desulfurizes sour hydrocarbons by contact with a suitable high surface area adsorbent. Before 1945, bauxite and Fuller""s earth were commonly used. Subsequent disclosures have taught the use of refractory oxides such as silica and alumina. More recently the use of certain molecular sieves has been disclosed. (Brooke, U.S. Pat. No. 3,051,646; Innes, U.S. Pat. No. 4,098,684).
Sweetening and/or desulfurizing a sulfur-containing hydrocarbon stream can also be achieved by contacting the stream with catalytic metals associated with porous supports. (Ferrara, U.S. Pat. No. 3,378,484; Ondrey, U.S. Pat. No. 3,783,125). Metals commonly used are generally found in Groups I-B, II-B, IV, V, VI, and VIII of the Periodic Table.
Finally, hydrodesulfurization desulfurizes hydrocarbons under conditions of elevated temperature and pressure by contacting the hydrocarbons with a large quantity of hydrogen gas in the presence of a special catalyst to form hydrogen sulfide. The hydrodesulfurization process has been widely used in oil refineries since about 1955 and is predicated on the availability of hydrogen gas at the refinery. Recently, however, it has been desired to treat hydrocarbons with higher sulfur concentrations resulting in an increased demand for the hydrogen gas required to treat hydrocarbons using this process. Thus, the readily available sources of hydrogen gas in an oil refinery may no longer be adequate for hydrodesulfurization. (Lapinski, et al., U.S. Pat. No. 6,013,598; Sakoda, U.S. Pat. No. 5,376,258).
The general object of this invention is to provide an improved process for sweetening and desulfurizing sulfur-containing hydrocarbon streams. The more specific objects are to provide a process which does not require the addition of metal reactants, does not require a phase separation of the hydrocarbons from an immiscible solvent, does not require the presence of a metal catalyst, and does not require the addition of hydrogen gas. Furthermore, this invention is directed at treating the crude feedstock, which could eliminate the need to treat individual products and make the feedstock safer for transportation and handling.
It has been discovered that these objects can be obtained by heating the sulfur-containing crude oil stream to a temperature of about 250xc2x0 F. (121xc2x0 C.) to 600xc2x0 F. (316xc2x0 C.) for about 1 to 4 hours. Simultaneously, an inert gas, such as nitrogen or carbon dioxide, is bubbled into the crude while the crude is being stirred. This process accelerates the removal of sulfur containing gases from the crude oil stream. After processing under these conditions, the hydrogen sulfide producing capacity of the crude oil is reduced significantly, thus making the crude safer for transportation and handling by reducing the health and environmental risks.
It has further been discovered that the addition of a polyalkylamine to the stirred crude oil can also greatly assist in the expulsion of sulfur containing gases. It has been found that these compounds can accelerate hydrogen sulfide formation in the crude oil approximately 5 to 15 times, further minimizing the sulfur emission problem of crude oil after processing.
In one embodiment of this invention, crude oil is sweetened and/or desulfurized by a thermal process that involves heating the crude to a relatively high temperature, 300xc2x0 F. (149xc2x0 C.) to 600xc2x0 F. (316xc2x0 C.), for a residence time of 1 to 4 hours.
Upon mild heating, hydrogen sulfide (H2S) has been shown to begin evolving from the crude oil. The production of H2S from crude oil that has been mildly heated is not a transitory phenomenon, as continued H2S evolution occurs over a very long period of time. Thus, the shipping and handling of crude oil can be dangerous. Sour crude will begin to produce H2S (in the vapor phase above the crude) as soon as any heating begins and will continue throughout the period of any mild heating. The sources of H2S from the crude are dissolved H2S and/or generated H2S produced by the reaction or decomposition of sulfur components in the crude. By pre-heating the crude to the above temperatures for up to 4 hours, the H2S producing capacity of the crude can be effectively reduced.
During the heating step it is preferred that the crude be stirred or mixed so that the heated crude can expel the H2S and other sulfur containing gases as they are either released or produced from the crude. The stirring rate can be anywhere between 10 and 5000 rpm which could be applied by a conventional stirrer or other mixing devices. An inert gas, such as N2 or a waste flue gas containing mostly CO2 (greater than 50%), may also be bubbled in to the crude to assist in the sulfur gas removal. Upon completion of this embodiment, there will be a dramatic reduction in the H2S and other sulfur containing gases from the crude, making the crude safer for transportation and handling, more environmentally friendly, and less likely to pose significant health risks to any workers around the crude.
The vapor that is produced from this heat treatment can be condensed to recover any condensable hydrocarbons that may also have been removed from the crude. If the crude has not yet been de-watered, a process by which the substantial amount of water found in crude oil is removed, the vapor that is produced from this heat treatment can contain water vapor. The vapor that is produced from this heat treatment may also be cooled to condense the water vapor.
Significant levels of hydrogen sulfides and other sulfur containing gases are released from crude oil if the crude is exposed to high temperatures for an extended period of time. Although thermal treatment alone can significantly reduce the sulfur containing gas production capability of a crude stream, high levels of H2S can persist even after a period as long as several days at elevated temperatures. These emissions can be controlled by limiting the heat applied to the crude after the initial thermal treatment, but where there may be an extended period of heating, additional steps will need to be taken to limit H2S production.
Another preferred embodiment of this invention involves sweetening and/or desulfurizing crude oil by adding catalytic additives to the crude. In this embodiment, the sulfur components of the crude are contacted with a polyalkylamine during thermal processing. The thermal processing step is similar to that of the previous embodiment, where the mixture of the crude and the catalyst is heated to a temperature of 250xc2x0 F. (121xc2x0 C.) to 600xc2x0 F. (316xc2x0 C.) for a residence time of 1 to 4 hours. The mixture of the crude and the polyalkylamine should be stirred or mixed during the thermal processing step so that the heated crude can expel the sulfur containing gases.
The addition of a polyalkylamine can accelerate H2S formation from sulfur containing components in the crude as much as 5 to 15 times. Preferred linear polyalkylamines are diethylene triamine (DETA), tetraethylene pentaamine (TEPA), and pentaethylene hexamine (PEHA). Preferred cyclic polyalkylamines are aminoethylpiperidine (AEP) and 1,4,8,11-tetra azocyclotetradecane (TACTD). The amount of polyalkylamine should be sufficient to accelerate H2S formation. The addition of a polyalkylamine preferably is between about 100 ppm and 3000 ppm, with a more preferred concentration of 0.2% by weight (2000 ppm).
It is believed that the polyalkylamines accelerate the release of sulfur containing vapors during heating of the crude oil by promoting the conversion of sulfur containing components into H2S and other sulfur containing vapors. This provides a means to both minimize subsequent H2S and other sulfur containing vapor emissions problems from the crude oil stream while simultaneously enhancing the crude value by removing elemental sulfur.
As in the first embodiment, the vapor that is produced from this thermal polyalkylamine treatment can be condensed to recover any condensable hydrocarbons that may also have been removed from the crude. If the crude has not yet been de-watered, the vapor that is produced from this thermal polyalkylamine treatment can contain water vapor and thus could be cooled to condense the water vapor.
The following examples are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples which follow represent techniques discovered by the inventors to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the scope of the invention.
Unless otherwise stated, all starting materials are commercially available and standard laboratory techniques and equipment are utilized.